Parasitic mode suppressor

ABSTRACT

A device for eliminating parasitic TE modes in a coaxial or cylindrical cavity, formed by two associated rings of electrically resistive material and which are axially movable. The rings have openings extending perpendicularly to the lines of force of the magnetic field of the mode which is to be suppressed. The dimensions of the slots are determined in such manner that they are resonant at the frequency of the parasitic mode.

United States Patent [72] Inventor Max Turteltaub Paris, France [2]] Appl. No. 23,332

[22] Filed Mar. 27, 1970 [45] Patented Jan. 11, 1972 I 73 I Assignee Thomson-CSF U] l'limlly MRI. 20,1969

| J 3 France 54] PARASITIC MODE SUPPRESSOR 2 Claims, 14 Drawing Figs.

[52] US. Cl 333/82 B, 333/83 A, 333/98 M [51] Int. Cl HOlp 1/16, HOlp 7/04, HOlp 7/06 [50] Field of Search 333/83 A [56] References Cited UNITED STATES PATENTS 2,456,653 12/1948 Snow etal. 333/83TX 2,593,234 4/1952 Wilson 333/83 A 2,605,459 7/1952 Cook..... 333/81 B 2,7l0,945 6/1955 Edson. 333183 A 3,087,089 4/l963 Niclas 333/83 X 3,475,707 10/1969 Feinleih 333/83 X 3,502,934 3/l970 Friedlunder et nl 3.33/83 X OTHER REFERENCES Harvard, Very High- Frequency Techniques" Vol. II, Staff of Radio Research Lab. Harvard U., McGraw Hill I947, pp. 9l l 914.

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorney-Kurt Kelman ABSTRACT: A device for eliminating parasitic TE modes in a coaxial or cylindrical cavity, formed by two associated rings of electrically resistive material and which are axially movable. The rings have openings extending perpendicularly to the lines of force of the magnetic field of the mode which is to be suppressed. The dimensions of the slots are determined in such manner that they are resonant at the frequency ofthe parasitic mode.

WVEA/Tu-R Y THTEL-TMJB PATEWEDJW 1 i572 Q 3634-790 sum 2 UF 3 NEW-m2 MM! TORTELTAUG PATENTED JAN! 1 1972 3,634,790

SHEET 3 OF 3 ODO\ m mam/rm MW TURTELTA'UB 87 f I A'EEJ T PARASITIC MODE SUPPRESSOR The present invention relates to improvements in devices for suppressing parasitic modes in the ultrahigh frequency field and to cavities and electron tubes equipped with such devices. The invention relates more particularly to the elimination of parasitic modes within cavities used as the input or output circuits of electron tubes.

In the ultrahigh frequency field, in order to avoid excessive radiation, the use of two-wire transmission lines is to be avoided. Accordingly, it is necessary to use screened transmission lines, coaxial lines or waveguides. The tuning circuits are then replaced by resonance cavities formed by terminated sections of waveguides or coaxial lines: termination is generally effected at one 'end by a short-circuiting piston which can move along the longitudinal axis of the cavity and whose distance from the. other end which is substantially equal to an odd number of quarter wavelengths, makes it possible to adjust the tuning as a function of the operating frequency.

In the case of a coaxial line, obviously the TEM mode is the one which is used, this mode being the dominant mode in this kind of circuit and having furthermore the advantage that it enables wideband transmission to be effected since there is no cutoff frequency. However, depending upon the setting of the tuning piston, it may happen that the frequency of the TEM mode is equal to the resonance frequency of a parasitic mode of higher order, for example the TE or TM,,,, modes, where the indices m and n designate whole numbers. The increase in losses, due to interference between the dominant mode and the parasitic mode, drastically reduces the output power from the associated transmitter or oscillator and thus gives rise to gaps in the range of usable frequencies.

In order to overcome this drawback, various methods have been used. Filters of the kind currently employed in prior art systems for the elimination of undesired modes, comprise conductive plates located perpendicularly to the lines of force of the electric field of the desired mode. Thus, in a circular waveguide for transmitting the TM, modes in which the electric field is radial, and eliminating the TE modes, a series of coaxial cylindrical plates are used. However, these plates have to have a substantial length in relation to the operating wavelength, and also their presence inside the waveguide conflicts with the operation of the tuning piston. In accordance with a more recent technique, attempts have been made to eliminate the T5,, mode by inserting in the coaxial cavity constituted by the electrodes of an electron tube, a ring of material having poor conductivity. However, this ring is fixed permanently at the time of construction of the tube and, while its operation may be satisfactory for a given frequency, if any attempt to vary this frequency is made, its efficiency tails off rapidly.

In certain cavities attempts have been made to modify the diameters of the internal and external conductors of the cavity, in order to increase the cutoff frequency of the TE wave. Generally speaking, however, in electron tube tuning cavities, the dimensions are in fact imposed by the tube dimensions so that this technique cannot be operated.

It is an object of the present invention to eliminate, within a cavity, the TB mode waves without substantially attenuating the selected mode of propagation. I

According to the invention, there is provided a device for suppressing parasitic TE modes in a coaxial or cylindrical cavity comprising a generally ring-shaped structure of an electrically conductive and resistive material, coaxial with said cavity, and having apertures whose plane intersects the magnetic lines of forces of the mode to be suppressed and means for displacing said structure along said cavity.

For a better understanding of the invention reference will be made to the drawing accompanying the ensuing description and in which:

FIG. 1 is an electron tube, a tuning cavity of which is equipped with the device in accordance with the invention;

FIGS. 2, 3 and 4 indicate the configuration of the electric and magnetic fields of the TEM wave in a coaxial cavity tuned to three-fourths of the wavelength;

FIGS. 5 and 6 show the configuration of the electric and magnetic fields of the TE mode, in the same cavity;

FIG. 7 shows the graphs indicating the resonance frequencies common to the TEM and TE, modes as a function of the tuning of the cavity;

FIGS. 8 and 9, show schematic sections of a preferred embodiment of the invention;

FIGS. 10 and 11 show schematic sections of another embodiment of the invention;

FIGS. 12 and 13 show schematic sections of still another embodiment of the invention; and

FIG. 14 shows a simplified section of a tuning cavity showing the relative dispositions of the suppressor ring, the shortcircuiting piston, and their respective control elements.

FIG. 1 illustrates, schematically and in section, a tube 10 with a cathode 11, a grid 12 and an anode 13, the tube being connected to operate as an amplifier. For the sake of clarity, the bias circuits and the cathode heater circuit have not been shown. The input circuits and output circuits are respectively formed by coaxial cavities I4 and 15. The grid-plate tuning cavity 15 is shown by way of nonlimitative example as folded around the cathode-grid cavity 14, which is a conventional arrangement in transmitter tubes for operation in the 1,000 MHz./sec. band. The cavity 15 is thus formed by a conductor 16, terminated by the face 17 which is aligned with the grid 12, and by a conductor 18 terminated by the front face 19 of the cavity. The input voltage is applied to the tube 10 by means of a capacitive probe 20 which penetrates into the gridcathode cavity 14 through an opening 21. Similarly, the output voltage is picked up by means of a capacitive probe 22 which penetrates into the grid-anode cavity through an opening 23 and is connected to the load (not shown). These two cavities is generally tuned to an odd number of quarter wavelengths by means of respective short-circuiting pistons 24 and 26 of annular form which can be moved axially by means of respective control rods 25 and 27. The mica capacitors 28 and 29, respectively inserted in the cathode and plate conductors, are intended to isolate the DC components of the cathode and anode currents.

As indicated hereinbefore, the dominant mode in this kind of cavity is the TEM mode, shown in cross section in FIG. 2 and in longitudinal section in FIG. 3. In these figures the full line arrows represent the electric field and the dotted arrows the magnetic field. The tuning of the cavity will be assumed to be set at 3M4, A being the operating wavelength. In a first approximation, the effect of the terminal wall 19 will be disregarded. The open end of the coaxial waveguide illustrated, corresponds to a voltage antinode where the electric field is maximum and the magnetic field minimum. The same condition is produced in the transverse plane 32 at a distance M2 from the open end. The end terminated by the short-circuiting piston 26 corresponds, conversely, to a voltage minimum and a current maximum; the same condition exists in the transverse plane 30, at a distance M4 from the open end. The wall 19 which closes off the cavity behaves substantially as a shunt capacitor across the open end of FIG. 3: the field is modified in the manner shown in FIG. 4 and thus axially acts on the electrons in transit between the grid 12 and the plate 13.

The parasitic mode most frequently encountered in a coaxial cavity is the TE, mode, this'being the mode having the lowest cutoff frequency which may exist in this kind of cavity. Although TE modes are likely to be permanently present in a coaxial line, given certain conditions, a particularly annoying situation occurs when there is resonance common to the TEM and TE modes at certain operating frequencies and tuning states of the cavity. The configuration of the TE mode is illuminated in cross section and in longitudinal section in FIGS. 5 and 6, where the full line arrows represent the electric field and the dotted arrows the magnetic field. FIG. 6, in particular, illustrates the property that the TE wave has of possessing a longitudinal magnetic field component. Many studies have been devoted'to resonance phenomena which are common to the TEM mode and the TE, mode in a coaxial cavity. Amongst the best known works, one should refer in particular to Microwave theory and techniques (Van Nostrand Company 1953, pgs. 473 ff), and to Very high-frequency techniques" McGraw-I-Iill 1947, vol. 2, pgs. 9i 1 ff), FIG. 7 shows the number of common resonance conditions which exist between the TEM mode and the TE mode as a function of the tuning of the cavity and the operating frequency. The abscissae plot the ratio LIA, between the length of the cavity and the length of the cutoff wave in the TE mode in the cavity; the ordinates plot the ratio )J), between the length of the operating wave and the length of the cutoff wave in the TE mode, The full line curves relate to the TEM mode. The parameters k=1, 2, 3 and 4, relate respectively to the tuning of the cavity to M4, 3M4, M4 and 7M4. The dotted graphs relate to the TE, mode. The parameters K ,=I, 2 and 3, respectively translate the corresponding tuning states of the cavity. It will be seen in FIG. 7 that, if the cavity is tuned to M4 for the TEM wave (K=1 there is no common resonance between the TEM mode and the TE mode. On the other hand, if the TEM wave is tuned to 3M4 (K=2), then it will be seen that there is resonance between this mode and the TE mode, this being demonstrated by the intersection between the curves K=2 and K,,=l.

For tuning of higher order, the curves show that there are progressively more and more common resonance conditions. In order to avoid these resonance conditions, it would therefore be preferable to select the M4 tuning state of the TEM mode, but for the higher values of the range this kind of tuning is no longer possible either because the short-circuiting piston comes up against the probe 22 or because it penetrates into the electrode space, depending upon the tube design, so that it is necessary to accept tuning to 3M4, 5M4, etc. In addition, even if there is no resonance condition for the TE wave, this wave may nevertheless under certain conditions of operation, give rise to the drawbacks enumerated hereinbefore and, consequently, it is highly desirable to provide for the suppression of this parasitic mode in the cavity.

FIG. 8 and 9 respectively illustrate in transverse and longitudinal section, a preferred embodiment of the invention.

The device 33 in accordance with the invention consists of a ring, made up of two elements, one external ring 34, shown in greater detail in FIGS. 10 and 11, and one internal ring 35,

shown in more detail in FIGS. 12 and 13. These two rings are fixed to one another and made of a material which, whilst conductive, has a high resistance to high-frequency currents. The ring 35 carries a series of concentric slots 36, six in the case illustrated, the length of which is substantially in the order of magnitude of the guided TE mode half wave. Actually, it is generally more expedient to make these slots somewhat shorter in order to take account of the capacitive effect of the walls, as described hereinafter. In addition to the half wave slots, the ring 35 contains three holes which serve to attach insulating control rods 38 (see FIG. 14). The outer diameter of the ring 34 is slightly smaller than the internal diameter of the wall 18 so that the ring 33 slides inside the cavity 15 without touching the wall 18. The lateral part of the ring 34, in the form of a cylinder, contains a series of substantially circular holes 39 having diameters substantially in the order of one tenth of the wavelength. In certain of these holes, insulating plugs 40, of Teflon for example, are inserted, one of which has been shown in the withdrawn position in FIG. 9. The plugs are designed to prevent any contact between the ring 33 and the wall 18 of the cavity, to center it mechanically therein, and to insulate it electrically.

It will be seen that during the operation of the tube, as far as the TEM mode is concerned, the lines of force of the magnetic field, shown in FIGS. 2, 3 and 4, are parallel both to the flat surface of the ring 35 and to the cylindrical surface 41 of the ring 34. Thus, no voltage is induced in the device and the TEM wave is not attenuated. As far as the TE wave is concerned, however, the distribution of the fields is different: the magnetic field of the TE wave has a component in the direction of propagation of the wave, i.e. normal to the surface of the ring 35, as FIG. 6 shows. These longitudinal loops of the magnetic field close through the half wave slots 36 and give rise in the ring 35 to an induced voltage which, in turn, produces a current which is dissipated in the form of heat by the resistive material of which the ring is made up. The slots 36, whose length is in the order of a half wavelength of the TE mode, behave as resonance slots so that the parasitic TE wave is considerably damped. FIG. 5 shows that a similar effect is obtained by the ring 34 whose lateral wall, containing the slots 39, is substantially normal to the lines of force of the magnetic field at its two maxima represented by the two points of convergence of the field in FIG. 5. In order to take account of the capacitive effect, produced by the walls, the slots 36 are slightly shorter than half the wavelength of the parasitic TE mode. Commencing from a cavity of given dimensions, the fundamental frequency of the TE mode of which can be calculated or measured, it is easy to determine the wavelength of the slots 36 and therefore the number of slots which can be cut in the ring 35. For the slots 39, contained in the lateral surface of the ring 34, the capacitive effect of the walls is substantially increased so that these slots must be shorter still than the half wavelength in order to give rise to the same resonance phenomena. Experience has shown that they can be given the form of circular orifices having a diameter in the order of one tenth of the wavelength.

The above reasoning shows the importance of placing the ring 33 in a zone corresponding to the maximum in the magnetic field associated with the TE mode. If the operating frequency is varied, so that the tuning of the cavity is modified too, the position of this maximum will vary as well and theoretically the position of the device 33 ought to be modified too. This adjustment can be effected by means of one of the rods 38 illustrated in longitudinal section in FIGv 14. The rod 38, assembled to the ring 33 by means of the holes 37, is of an insulating material and slides with no friction through the corresponding openings formed in the short-circuiting piston, the latter being operated in all cases by the control rods 27. The displacement of the ring 33 by the rods 38 thus makes it possible to find the optimum position for producing maximum attenuation of the parasitic mode. In practice, experience has shown that once this position has been found, the operating frequency can be varied by an amount in the order of 50 MI-Iz./sec. without any necessity for readjustment of the ring 33.

Experience has shown that in certain cases, it may be desirable to reduce the ring 33 to a rim 34 or to a rim 35, one or the other being operable by means of rods such as the rods 38. In particular, if at high frequencies in the operating range, it is desired to tune to M4 or 3M4, then the rim 35 turns out to be extremely effective because of its small thickness.

Among the materials which can be used to produce this kind of device, it is advantageous to employ ferromagnetic alloys such as ferronickel alloys, kovar, and so on, in order to avail ones self of the skin effect at high frequencies. Also, components of nickel-plated copper have been used with advantage, this system producing both the desired skin effect and a satisfactory thermal gradient.

The following data, given by way of example in order to illustrate the foregoing description, relate to the tuning cavity of a tube operating at frequencies in the order of 460860 MHz./sec. i.e. in the bands 4 and 5. The coaxial cavity was defined by two diameters 142 mm. and mm.; a ring 33 was used in which the rim 34 had a maximum diameter of 141.4 mm. and the rim 35 an internal diameter of mm. Each of the six resonance slots had a length in the order of l 18 mm. and a width of 12 mm. In operation, for an anode voltage of 3,500 v. and a screen voltage of 500 v., the TE mode pulled in at a plate current of 500 ma. without the mode suppressor. Using the ring 33, however, an anode current of 5 a. could be obtained.

If the electric and magnetic fields in the various waveguides are considered it will readily be appreciated that a device 33 may be used quite both for the elimination of higher order TE modes, such as TE TE etc. in a coaxial cavity, or for the ture of an electrically conductive and resistive material. coaxial with said cavity and having apertures whose plane intersects the magnetic lines of forces of the mode to be suppressed and means for displacing said structure along said cavity, in which the ratio of the major dimension of said apertures to the half wavelength of the mode which is to be suppressed, is such that said apertures are resonant for this mode.

2. A cavity resonator comprising a device as claimed in claim I.

i i t i i 

1. A device for suppressing parasitic TE modes in a coaxial or cylindrical cavity comprising a generally ring-shaped structure of an electrically conductive and resistive material, coaxial with said cavity and having apertures whose plane intersects the magnetic lines of forces of the mode to be suppressed and means for displacing said structure along said cavity, in which the ratio of the major dimension of said apertures to the half wavelength of the mode which is to be suppressed, is such that said apertures are resonant for this mode.
 2. A cavity resonator comprising a device as claimed in claim
 1. 